If an archaeologist tells you that he has unearthed some plastic buckets and trinkets made from aluminium and magnesium in Ancient Egyptian sites will you believe him? Why?

Not unless aliens gave those Ancient Egyptians a helping hand in building buckets and trinkets as well as designing the pyramids for them!

Although organic polymers (essentially naturally occurring plastic) was used by ancient civilizations, magnesium wasn't isolated until 1808.

It is abundant in the Earth's atmosphere, but not in its free form.

As for aluminium, the Greeks and Romans used it in salt form as a dyeing agent. However, it didn't exist in metal form until 1825.

So, either the Ancient Egyptians were working with an advanced alien race, or your archaeologist friend is lying to you.